Automatic drive trains including automatic transmissions that are shifted under power and that are operated by means of actuators based on preset programs are increasingly being used in automobiles. Transmissions of this type not only increase driving comfort but they also enable substantial fuel consumption savings as the amount of driving done at lower engine speeds is increased.
FIG. 1 shows a section of a drive train of a vehicle that is equipped with such a transmission. A twin-clutch transmission, indicated as a unit by the number 10 and also called a parallel manual transmission, has two input shafts 12 and 14, each of which can be connected via a clutch 16 and 18, respectively, to the crankshaft 20 of an internal combustion engine. Gear wheels are arranged on the input shafts 12 and 14 and can be meshed with gear wheels that are arranged on an output shaft 22 such that the gear wheels can shift but not rotate, in order to allow shifting to different gears. The output shaft 22 is connected, for example, to a rear differential of the vehicle.
One actuator 24 or 26 is allocated to each clutch 16 or 18, respectively, to implement its actuation.
The actuator 26 is illustrated here in detail by way of example and includes an electric motor 30. The output pinion gear 32 of the electric motor 30 is equipped with internal threading that meshes with a threaded tree rod 34, which is at the same time the rod for a piston 36, which operates in a hydraulic cylinder 38. The hydraulic cylinder 38 is connected via a hydraulic transmission link 40 to an actuating mechanism, for example a release lever, of the clutch 18. To ascertain the position of the piston 36 or the release lever of the clutch 18, a sensor 42, such as an increment counter, is used to determine the angle of rotation of the output pinion gear 32. Because of the threaded engagement between the pinion gear 32 and the threaded tree rod 34, the linear shifting of the threaded tree rod 34 can be ascertained from changes in the angle of rotation. In order to have a point of reference that is referred to in the shifting of the piston 36, a detector bore provided in the hydraulic cylinder 38 can be passed over in a known manner, while pressure builds up in the hydraulic transmission link 40, by an increase in torque or in the energy input of the electric motor 30. As additional reference points, centers of pressure or mechanical stops for the clutch can be approached.
The gear wheels arranged on the output shaft 22 of the twin clutch transmission 10 are moved by means of selector forks 46, which operate in conjunction with selector fingers 48, which are arranged on a shifting shaft 50. For example, a shifting shaft 50 having a number of selector fingers 48 may be provided, which actuates all the gear wheels arranged on the output shaft 22, or, for example, two shifting shafts 50 may be provided, which actuate the selector forks of those gear wheels that are allocated to one of the input shafts 12 or 14. For each shifting shaft 50, two actuators are provided, one that rotates the shifting shaft 50 back and forth around its axle to engage gears, and another that shifts the shifting shaft 50 in accordance with the drawing, perpendicular to the plane of the paper, to choose between various shifting tracks.
The actuators that are allocated to a shifting shaft 50 (also indicated by “i”) are indicated in the figure by the numbers 52 and 54. The position sensing can be executed as described above in reference to the electric motor 30. In order to determine the absolute position of the shifting shaft 50 or the selector finger 48, reference points are approached, for example mechanical stops in a selector track or shifting track, or mechanical stops approached by the selector forks 46 themselves.
To control the above-mentioned actuators (24, 26, 52, 54), a control unit or control device 60 is provided, which includes a microprocessor 62 with a program memory 64 and a data memory 66.
Inputs 68 to the control unit 60 are connected to various task-based sensors or position indicators, such as a wheel speed sensor 70 for determining the speed of a wheel, a sensor 72 for determining the position of an accelerator pedal 73, a position indicator 74 for determining the position of the selector lever of a transmission actuating device 76, an output 77 of the transmission actuating device 76, through which the actuation of various control programs, for example a comfort driving or performance driving program, is transmitted, a speed sensor 78 for determining the speed of the internal combustion engine, etc. It is understood that the control unit 60 may also be designed such that it can itself recognize driving conditions and/or driver profiles and can activate corresponding programs, such as a mountain driving program or a performance driving program.
Outputs 79 from the control unit 60 are connected to the actuators, which can be actuated by another actuator, and to a powershift element 80 of the internal combustion engine.
The design and the function of the above-described arrangement, which can be altered in a multitude of ways, are known in the art and thus will not be described in greater detail.
One problem with this type of automatically actuated parallel manual transmission, or with automated manual transmissions in general, lies in the fact that under certain conditions with an acceleration reference, it is not possible to achieve the rates that can be achieved with a manually operated manual transmission.
This is due generally to the fact that with a manually shifted transmission, during acceleration measurement the vehicle operates outside of a permissible range for individual components, for example with extremely high shifting forces, a “smoking” clutch, etc. With automated manual transmissions, this is not possible since otherwise the danger would exist that frequent repetitions of such acceleration attempts could destroy components of the drive train.
With automated manual transmissions that are shifted with an interruption in propulsive power, during the shifting process the engine speed is limited by means of ignition retard, or directly in the powershift element, as otherwise the engine would race uncontrollably during the shifting process.
In a vehicle having a parallel manual transmission or twin clutch transmission there is no need for slow gear shifts or a load-reducing ignition retard in order to avoid a racing of the engine during the shifting process, because the next gear in the shifting process can be selected ahead of time by engaging a gear wheel of the input shaft that is being operated with an open clutch, with a gear wheel of the output shaft, and by effectively engaging the gear wheel by simply switching the clutches 16, 18. Nevertheless, with upshifts following the end of the overlapping phase of the two clutches 16, and 18, a load-reducing ignition retard is executed in order to lower the engine speed without superelevating the output torque to the target speed. As a result of this load-reducing ignition retard, the available propulsive power is not fully utilized, causing acceleration time to be wasted.
One property of automatic transmissions consists in the fact that these transmissions, if necessary, automatically downshift to a lower gear or transition to a higher gear if the propulsive power is no longer sufficient to fulfill the wishes of the driver. One characteristic feature of powershift transmissions, especially parallel manual transmissions, but also of conventional automatic transmissions that operate with planetary gear sets, or CVT [continuously variable transmission] transmissions (transmissions with continuously variable gears), is that during the transition to a higher gear the engine power is used both to increase the engine speed and to propel the vehicle, which can have a negative effect on driving comfort.